Electrostatic direction of exploded vapors



Oct. 26, 1965 R. G. LINS ETAL.

ELECTROSTATIC DIRECTION OF EXPLODED VAPORS Filed March 5. 1962 INVENTORSRAYMOND G. L/MS ERW/A/ 6. WEBER United States Patent 3,213,826ELECTRGSTATIC DIRECTION OF EXPLUDED VAPGRS Raymond G. Lins, Minneapolis,and Erwin G. Weber, Hopkins, Minn, assignors to Sperry Rand Cor oration,

New York, N.Y., a corporation of Delaware Filed Mar. 5, 1962, Ser. No.177,678 3 Claims. (Cl. 118-491) The present invention relates generallyto the deposition of thin films, and more particularly to the explosiveevaporative and controlled deposition of thin film-s of a magneticmetallic material onto a suitable substrate or base member.

The present invention is an improvement over that certain inventiondisclosed and claimed in Patent No. 3,142,- 587 of Erwin G. Weber,Serial No. 80,301, filed January 3, 1961, and entitled Apparatus forProducing Electrical Conductor Films by Explosive Evaporation. In thatpatent, the explosive evaporation and consequent deposition of magneticthin films is disclosed. However, in connection with the technique ofthe present invention means are provided for steering, guiding, andcontrolling the flow of the evaporant from the source or explosion locusto the ultimate deposition of the evaporant upon a suitable substratemember.

In the past, various ways have been utilized to prepare magnetic thinfilms, particularly thin films for use in connection with binarymagnetic memory devices. For example, certain of these techniques beingset out in detail in Rubens Patent No. 2,900,282. In that patent,reference is made to a vacuum deposition of thin magnetic films whereinthe evaporation operation is accomplished by inductively or otherwiseheating a source of ferromagnetic material and causing the vapors tocondense onto a substrate. In the present invention, magnetic ornonmagnetic films may be prepared by deposition in vacuum or in anon-contaminating atmosphere onto a suitable substrate by explosivelyevaporating a conductive body which is preferably resistively preheated,means being provided to steer, guide, or otherwise control thetransmission of the material from the locus of the explosive operationonto the surface of the substrate.

Explosive evaporation oifers the advantages of: (1) a short depositiontime of less than 1 sed, which reduces the interaction between the metalvapor and the residual atmosphere in the vacuum to a minimum; (2)absence of contact between the material to be evaporated and othermaterial-s all of which may be held at high temperatures (for example,crucibles, boats, etc.) prevents alterations of the material or of thevaporization process; (3) rapid heat generation prevents fractionaldistillation; (4) short deposition time minimizes interaction of thecondensing vapor with the substrate (diffusion, chemical reaction,etc.); and (5) exact calibration of the evaporated total mass necessaryis readily carried out by a simple length measurement of the conductorexploded. In addition, explosive evaporation offers the substantialadvantage of making it possible to evaporate binary, ternary, or higherorder alloys without regard to the individual vapor pressures at theevaporation temperatures which are employed. Conventional evaporativedeposition of these higher order alloys is difficult to achieve due tothe differences in vapor pressure at the evaporating temperatures, sincethe composition of the individual films will vary from the initialevaporant reaction to the composition of the later evaporated materials.When magnetic thin films are being prepared for use in connection withmemory elements, certain compositions are undesirable inasmuch asmagnetostrictive effects may develop in the material causing differingmagnetic switching characteristics to occur, both ice of which have adeleterious effect upon the use of these materials as magnetic memoryelements.

In utilizing explosive evaporative techniques in the preparation of thinfilms, a greater degree of uniformity with regard to deposition across arelatively large crosssectional area along with other features may beachieved when a mechanism for steering or otherwise controlling the flowof material from the locus of the evaporation to the substrate isemployed. The steering means or mechanism is the arrangement of a sourceof an electrostatic field or electromagnetic field or combinationthereof at a point which is remote from the location of the substrate.The material which is to be explosively evaporated is preferablydisposed at a location which is along a line between the source of thesteering means and the substrate surface.

It is, therefore, the object of the present invention to provide amethod and apparatus for controllably efiecting explosive evaporation ofan electrical conductor.

It is a further object of the present invention to provide means forpreheating the conductor to be evaporated and to provide at least aportion of the energy required to vaporize the conductor by thepreheating operation.

It is yet a further object of the present invention to provide asteering means for the evaporant, the steering means being provided bythe presence of an electrostatic and electromagnetic field due to thecharging of the energy transmission portion of the explosive evaporatingsystem and the flow of energy through the system and the conductor beingevaporated, the energy transmission means being disposed adjacent to theconductor being evaporated and in sufliciently close proximity theretoto have an influence upon the evaporant medium.

Other and further objects of present invention will become apparent tothose skilled in the art upon a study of the following specification,appended claims, and accompanying drawings in which,

FIG. 1 is a front plan view of apparatus specifically adapted forcarrying out the technique of the present invention, the enclosure beingshown in section, and further showing a schematic illustration oftypical circuitry which may be employed in connection with the explosiveevaporative operation;

FIG. 2 is a horizontal sectional view taken along the lines and in thedirection of the arrows 22 of FIG. 1; and

FIG. 3 is a partial plan view of the preheat portion of the systemutilized in connection with the operating cycle, certain componentsbeing shown in section.

Referring now to a preferred embodiment of the present invention as isillustrated in the accompanying drawings, the evaporating systemgenerally designated 10 includes a base platform 11 which is surroundedby a suitable bell-jar enclosure or the like 12. The platform 11together with bell-jar 12 is preferably fabricated from a non-magneticmaterial in order that any tendency toward generation of stray magneticfields which may have an adverse effect upon the preparation of thefilm, may be completely if not substantially entirely eliminated. Within the base 11, an evacuating tube 14 is provided, the tube 14communicating with the interior of the bell-jar 12 and permitting properevacuation of the jar. In the alternative, tube 14 provides access forintroducing a desired inert atmosphere to the enclosure.

Referring now to the apparatus which is adapted to accomplish theexplosive evaporation of the desired material, a suitable Wire, bandstrip, or the like such as the wire 16 fabricated from a nickel-ironalloy comprising 82% nickel, balance iron, is maintained between theretaining members 18 and 19, these retainers being adapted to maintainthe conductor 16 in a properly taut relationship. Conductive leads andsteering field generating means consisting of the rods 21-21 areprovided in order to transmit electrical energy at the desired level tothe conductor 16. The leads 2121 are insulatingly supported within thebase 11 through the insulating grommet 22. The substrate supportingbracket 24, is secured by any desired means to the base plate 11, thebracket 24 including a pair of brackets 25-25 thereon which are adaptedto hold or retain the substrate member 26 in a predetermined positionrelative to the conductor 16 and to the steering field generating means2121.

Referring now to the schematic illustration of the circuitry employed inthe system, two phases of the operation are indicated, the first beingthe preheat mechanism which includes a heater power source 30 togetherwith controlling switch 31, and a fusing power source 32 together withcontrolling switch 33. The main explosive power is provided by the highvoltage D.C. source 35, this source being connected across capacitor 36.Charging switch 38 is provided in the capacitor charging circuit alongwith the charging resistor 39. Upon completion of each of thepreliminary steps prior to the explosion operation, per se, switch 40 isadapted to complete the circuit from the power source through main leads21-21, upper ball 18, the conductor to be exploded 16, the the supportball and rod 19 and thence through the base plate 11 to ground.

The equipment and circuitry which is adapted to operate the preheatcycle is arranged to be disconnected from the remaining phases of thecircuitry at the completion of the heating cycle, thereby being isolatedtherefrom. Suitable hold-down schemes are utilized in order to providethe appropriate disconnect for this operation. With particular attentiondirected to FIG. 3 of the drawings, it will be seen that preheat post 45is provided adjacent upper rod 21, this post being insulatingly disposedwithin the base plate 11 and adapted to conduct preheat current into theconductor to be exploded 16. Spring biased conductor or line 46, held inbiased position by the resilient member 47, is adapted to carry currentto the half-round connector 49, which is held in electrical contact withthe upper portion of conductor rod 21. Halfround connector 49 isarranged to be physically separated and disconnected from the rod 21when desired, this separation being accomplished by means of the fusingarrangement disposed to the right of the preheat arrangement. The fusingportion includes the post 50, the resilient conductor 51, the fuse orfusible element 52 which is attached or secured to the clamp 49 by meansof the conductor element 53. Upon completion of the preheat cycle,switch 33 will be closed thereby energizing the 'fusing portion of themechanism, the current being sufficient to rupture fuse element 52 andthereby permit the connector 49 to be displaced from the remainingportions of the mechanism. Resilient member 47 will carry the connector49 therealong and will accordingly perform the disconnect operationaround the rod 21.

In a typical explosive operation adapted to be carried out in connectionwith the apparatus shown in the drawings will now be described.

The atmosphere within the bell-jar 12 is either evacuated or filled witha suitable inert gas. The pressure is preferably between about 1 l mm.Hg and 1x10- mm. Hg and there is no minimum pressure limit. In theinitial step, the operator closes switch 31 which initiates the preheatcycle. The preheat proceeds for a predetermined time, that is, theconductor to be exploded is held at a sufi'iciently high temperature fora sufficiently long period of time as determined either by practice orthrough observation with suitable temperature measuring devise such asan optical pyrometer or the like. After the preheat cycle has beencarried out for a suitable length of time, switch 33 is closed whichprovides for a surge of current through fusible element 52, the element52 separating and permitting the contact element 49 to be removed fromphysical contact with the conducting rod 21.

Switch 38 has previously been closed, this permitting charging of thecapacitor 36 from source 35. Upon completion of the charging operation,as determined from the requirements of the explosive operation, switch40 will be closed so as to enable a full surge of electrical energy tobe passed through the system the quantity of energy being adequate andavailable at such a rate so as to explosively evaporate conductor 16.Switch 38 is preferably closed just prior to closing of switch 40.

The high voltage and energy surge from the discharge of capacitor 36(which may be a bank of condensers totaling six microfarads of capacity,for example) may be effected by previously charging in any desiredmanner, such as, for example, by a 110 volt source, which is operativelyassociated With a suitable high voltage charging source. The capacitor36 is charged by the high voltage charging source through the highresistance (9 megohms, for example) resistor 39.

When conductor 16 explodes upon the application of the high voltagetogether with the high electrical energy, the vapor which is generatedas a result of this explosion, as steered, guided or the like along thedirection of the arrows, where indicated upon the surface of thesubstrate member 26. If desired, the substrate 26 may be masked, asrequired.

In connection with certain materials being evaporated and deposited, itis desirable that the preheating temperature of the conductor to beevaporated be held below the melting point thereof, although thetemperature is preferably sufiiciently high so as to be immediatelybelow the melting point. The purpose of the preheating cycle is tosupply a portion of the energy which is required to accomplish theexplosive operation. For an -20 Permaloy layer having the length of fourinches, 20 mils diameter, the optimum preheating temperature is about1000 C. At amuch lower temperature the amount of heat generated in thewire by the discharge energy of the system is too small to completelyevaporate the wire, and with higher temperatures, the evaporation is notuniform because of apparent weak spots in the surface tension along theelement. It has been found that greater uniformity results with atemperature held at about 1000 C. with this wire. With a temperature ofabout 1000 C. the four inch wire as indicated herein above forms filmsof about 500 Angstroms thickness on a substrate which is located oneinch from the wire source.

The preheating temperature may be maintained for a period of about 10minutes, or longer if desired, in order to permit outgassing to occur.The elevated temperature should not be maintained for such a period oftime so as to enable oxidation to occur along the surface of the wire.As between a plurality of 8 mm. diameter circular Permalloy filmsdeposited onto ordinary glass microscope slides, a remarkable uniformitywith regard to the type of hysteresis loop, coercive force andanisotropy field was noted. For most films the coercive force was in therange of from 2-10 0e. and the anisotropy field was always greater than0e. Uniformity from one evaporating Wire charge to the next can beexpected if the wires have uniform characteristics, if the preheattemperature is maintained substantially the same, and if the operationcycle is carried out in a uniform manner. For example, a recommendedtime lapse between cessation of the preheating cycle and thecommencement of the explosion cycle may be about 0.1 second. Times inexcess of this period are not recommended since the degree of uniformitybetween products may suffer. However, it should be pointed out, thatlonger periods may be utilized provided they are uniform and providedthe rate of cooling of the wire is maintained substantially uniform. Inthis regard it will be appreciated that the thermal gradients along thelengths of the wire are much greater when the preheat cycle is notcontinued to the extent that a certain constant temperature is achievedat the contact balls 18 and 19. These gradients along the wire arepreferably avoided inasmuch as they may encourage splattering of theWire onto the substrate due to certain portions of the wire not havingreached a sulficiently high temperature to become completely vaporizedduring explosion.

The voltage to which the capacitor 36 is charged is preferably in therange of kv., however, this value may be varied considerably dependingupon the conductive characteristics of the particular wire or conductorbeing exploded. It has been found that 20 kv. is sufiicient to explode81l9 Permalloy wire which is 6" long and mils in diameter or which is 4"long and mils in diameter. There is no apparent maximum value for thehigh voltage other than the limitations imposed by the equipment beingutilized, and the minimum voltage requirement is that determined by thecharacteristics of the material being evaporated.

The steering or guiding field is set up by the vertical and diagonalconductors or rods 2121. The fields which are set up must be, of course,disposed adjacent from the substrate with the conductor to be explodedbeing arranged between the substrate and the feed-through lines. Thefield is believed to be both an electromagnetic and electrostatic, theelectromagnetic field being generated during the period of time thatcurrent is flowing along the conductors. Indications are, however, thatthe greatest influence on the plasma or evaporant is due to anelectrostatic field rather than electromagnetic, however, the compositeinfluence is believed to be a combination of the two types of fields.

In accordance with the present invention, it has been found that the useof a guiding, deflecting or steering field provides a greater degree ofuniformtiy between the magnetic films so prepared. In this regard, filmshaving a greater degree of uniformity, one to another, as well as agreater degree of magnetic uniformity, one to another, are prepared withthe use of the steering field as compared to those prepared without theuse of the steering field. When an explosive evaporation of a metallicsubstance occurs, and when there is no electrostatic field or the likepresent to direct the film forming substance in any one direction, thedistribution of evaporant will be substantially random. When a steeringfield such as an electrostatic or combined electrostatic orelectromagnetic field is provided the evaporant may be carefully andcontrollably directed onto the surface of a substrate member. In thisconnection, it is possible to explosively evaporate electricalconductors having a mass which is substantially closely related orcorrelated to the mass of the film desired. It is accordingly notnecessary to evaporate large quantities of superfluous evaporant when asteering field is available to control the flow of evaporant from thelocus of the explosion to the surface of the substrate. Therefore, theminimum requirements of the mass of the film being prepared may becompatible with the maximum requirements of the means for providingexplosive evaporations. Inasmuch as relatively large quantities ofenergy must be available to perform explosive evaporation operation ofelectrical conductors, the presence of a steering field renders theenergy requirements more modest, and hence compatible with the equipmentavailable for performing the task.

It will be appreciated that the apparatus and techniques of the presentinvention as set forth in this application, are exemplary only and arenot to be construed as any limitation upon the scope of the inventiondisclosed in claims herein. It will accordingly be understood that thoseskilled in the art may depart from the examples illustrated here andwithout actually departing from the spirit and scope of the presentinvention. Having now, therefore, fully illustrated and described ourinvention, what we claim to be new and desire to protect by LettersPatent is:

1. Apparatus for depositing a film on a substrate member comprising:

an evacuatable enclosure, said substrate member being disposed withinsaid enclosure,

an electrical conductor disposed within said enclosure adjacent saidsubstrate member,

evaporating means operatively associated with said conductor forproviding an evaporant by explosively evaporating said conductor, and

means for generating a steering field with resultant force componentssufficient to direct substantially all of said evaporant generallytoward said substrate.

2. Apparatus as in claim 1 wherein said evaporating means includes saidgenerating means.

3. Apparatus as in claim 2 wherein said electrical conductor is disposedat a location which is along a line between said evaporating means andsaid substrate member.

References Cited by the Examiner UNITED STATES PATENTS 2,357,415 9/44McManus Q 118-48 X 2,382,432 8/45 McManus 118 18 X 2,975,332 3/61 Starr315-169 2,976,174 3/61 Howard 11793.2 2,996,418 8/61 Bleil 117-9323,142,587 7/64 Weber 117-107 FOREIGN PATENTS 702,937 1/41 Germany.

JOSEPH B. SPENCER, Primary Examiner.

RICHARD D. NEVIUS, Exa iner.

1. APPARATUS FOR DEPOSITING FILM ON A SUBSTRATE MEMBER COMPRISING: ANEVACUATABLE ENCLOSURE, SAID SUBSTRATE MEMBER BEING DISPOSED WITHIN SAIDENCLOSURE, AN ELECTRICAL CONDUCTOR DISPOSED WITHIN SAID ENCLOSUREADJACENT SAID SUBSTRATE MEMBER, EVAPORATING MEANS OPERATIVELY ASSOCIATEDWITH SAID CONDUCTOR FOR PROVIDING AN EVAPORANT BY EXPLOSIVELYEVAPORATING SAID CONDUCTOR, AND MEANS FOR GENERATING A STEERING FIELDWITH RESULTANT FORCE COMPONENETS SUFFICIENT TO DIRECT SUBSTANTIALLY WALLOF SAID EVAPORANT GENERALLY TOWARD SAID SUBSTRATE.